"554 delivery error: dd This user doesn't have a yahoo.com account ([recipients' userid]@yahoo.com)" - The email didn't go through because the recipient's user ID doesn't exist. You shouldn't retry the message, as it will never complete successfully.

"421 Resources temporarily unavailable. Please try again later" - The Yahoo Mail servers are temporarily too busy. Your server should try to resend the message after a few minutes.

If you don't see these, or similar error messages, the emails in question aren't being blocked or deprioritized. You should see an SMTP confirmation code similar to "250 ok dirdel" that indicates your message has been delivered.